Its a simple question with either a simple answer of just see what works (trial and error) or a complicated answer if you actually want to work out what you need.
Just how much power are these ancillaries and the rest of the vehicle electrics using? How much night time laneing do you do with the sound system at full volume? You will likely find that the power consumption varies a lot and very little of the time is spent at full output. Also I expect that the 500w is max output, if so then ingore it and use the amps RMS output.
Is it power consumption more than the alternators output for a significant amount of time? If yes then your on borrowed time.
Once you exceed the alternator output and start discharging the battery its voltage will drop steadily, the lights won't be as bright and the stereo won't play as loud.
The life expectancy of the battery depends on how much you discharge it and how often. Discharging it a lot just a few times will have the same effect as discharging it only a little a lot of times. This damage to the life expectancy will be cumulative. The more times you discharge it and deeper you discharge it the sooner it will die.
